Thermal imaging systems are often employed to detect fires, overheating machinery, planes, vehicles and people, and to control temperature sensitive industrial processes. Thermal imaging systems generally operate by detecting the differences in thermal radiance of various objects in a scene and by displaying the differences as a visual image of the scene.
The basic components of a thermal imaging system generally include optics for collecting and focusing thermal radiation from a scene, a thermal detector having a plurality of thermal sensors for converting thermal radiation to an electrical signal, and electronics for amplifying and processing the electrical signal into a visual display or for storage in an appropriate medium.
The thermal sensors of a thermal imaging system may be disposed in a focal plane array. The focal plane array and its associated thermal sensors are often coupled to an integrated circuit substrate with a corresponding array of contact pads and a thermal isolation structure disposed between the focal plane array and the integrated circuit substrate. The thermal sensors define the respective picture elements or pixels of the resulting thermal image.
One type of thermal sensor includes a thermal sensitive element formed from pyroelectric material which exhibits a state of electrical polarization and/or change in dielectric constant dependent upon temperature changes of the pyroelectric material in response to incident infrared radiation. A pair of thin film electrodes are generally disposed on opposite sides of the pyroelectric material to act as capacitive plates. In this arrangement, the pyroelectric material acts as a dielectric, or insulator, disposed between the capacitive plates. Accordingly, the electrodes are operable to measure the charge generated by the pyroelectric material in response to changes in temperature. As previously discussed, the charge, or electrical signal, may be amplified and processed into a visual display.
A chopper is often included in a thermal imaging system to A.C. couple the detector to the scene. The chopper produces a constant background radiance which provides a reference signal. The electronic processing portion of the thermal imaging system will subtract the reference signal from the total radiance signal to produce a signal with minimum background bias.
A problem with choppers, however, is that they are relatively expensive to manufacture. Additionally, choppers often generate a background that is not uniform or that greatly differs from the energy of the scene. A background that is not uniform may prevent adequate normalization of the thermal sensors. A background that differs from the scene may expose the thermal sensors to a large dynamic range that degrades quality of the image obtained from the scene.